1. Field of the Invention
The invention is in the field of data storage and of micro-mechanical systems. It more particularly relates to a method of structuring a surface, a memory element, a storage medium as data carrier means, a data storage device, and a use of method of structuring a surface.
2. Description of Related Art
Due to increasing miniaturization in many technological fields, including data storage, techniques for applying specific microstructures in the sub-micron or micron range to surfaces become increasingly important. An interesting technology in this context is also the Micro Electro Mechanical Systems (MEMS). Another promising application field concerns the silicon-on-insulator (SOI) technology.
Known techniques for structuring and probing surfaces in the sub-micron region include Scanning Probe Microscopy (SPM), e.g. Atomic Force Microscopy (AFM) or Scanning Tunneling Microscopy (STM). These techniques serve as tools for investigating materials as well as for finding new materials or methods for the application in storage media.
An example of a storage device based on SPM is disclosed in U.S. Pat. No. 5,835,477. A device including arrays of SPMs is described. Perturbations on an essentially flat storage medium surface serve as data bits. Due to the multitude of SPMs, data bits can simultaneously be generated, erased or read. Examples of perturbations serving as data bits include indentations in a soft material as well as locally modified chemical properties, domain structures, crystalline phases, polarization states, and electronic states. All the disclosed perturbations have in common that they are easily disturbed and that they have a rather limited lifetime. Especially, indentations in a wax like medium are relatively quickly smeared out by the mechanical wear of the reading process.
MEMS sensors and actuators become increasingly important. These systems are usually based on a microstructure of silicon and combine mechanical and electrical functions.
SOI (silicon on insulator) technology has recently gained considerable importance. SOI devices comprise a silicon oxide layer and a thin silicon layer on top of it. In this silicon layer, the transistor structure of a chip is formed. Compared to conventional chips built atop a crystalline silicon wafer, the charge-up process when a transistor is switched is done more quickly and efficiently and less power consuming.
When a SOI chip is built, in a first step the silicon-on-oxide structure has to be produced. To this end, two technologies are available: SIMOX (silicon implanted oxide) and bonded SOI. Both technologies have in common the disadvantage that the structure quality and, especially, the interface quality are difficult to control. It would therefore be desirable to have a method and a device that allow testing of the interface quality of a silicon/silicon oxide interface.
It is an object of the present invention to provide a memory element that produces a reliable information bit with a long lifetime.
It is a further object of the present invention to provide a new method for structuring a surface on a micron or submicron scale that overcomes drawbacks of existing methods and that especially reliably produces a structured surface with information bits with a long lifetime. Preferably, the bits should be readable repeatedly an arbitrary number of times.
It is also an object of the invention to provide a storage medium and a storage device as well as a use for the method of structuring surfaces.
It is a further object of the invention to provide a method for manipulating a surface structure on a submicron or micron scale.
A basic memory element according to the invention is made up of a protrusion on a substrate. The protrusion is such that a predetermined breaking point is present. The protrusion is removed when a pre-defined force is applied onto it. The memory element in this way serves as data bit: the information is given by the fact whether the protrusion is broken off or not.
The invention is based on the insight that an interface a protrusion with a predetermined breaking point can serve as a non-volatile memory element. The protrusion is removed by simply applying a predefined force, such as a lateral force, on the protrusion.
An interface, for example, involving two different materials, can serve as a well defined area of fracture and, thus, as predetermined breaking point. Further preferred predetermined breaking points include breaking points given by a protrusion""s geometry, for example its base or constrictions at arbitrary elevations above.
An idea underlying the invention is to make use of the non-volatile character of this information storage. Two examples:
A plurality of basic elements can make up a storage medium. A surface is structured by starting from a first structure, preferably a regular pattern of protrusions, and by selectively removing protrusions by a micro-device. The resulting pattern can be used to code information/data and therefore is an example of an extremely reliable ROM (read only memory) that can not be manipulated.
A basic memory element according to the invention can be combined with a micro electro mechanical sensor. Such a sensor may be formed as a protrusion with a sensor element. The sensor element causes the protrusion to be bent as a function of the quantity to be measured. If the protrusion is at the same time formed to be a memory element according to the invention, the protrusion is broken off if the measured quantity exceeds a certain value. The data bit contains then non-volatile information about the fact if the quantity has once been exceeded. In this way, a concept of xe2x80x9cthreshold securityxe2x80x9d is introduced.
The method for structuring a surface is essentially characterized in that in a first step a substrate of a first material is provided, which is coated with a defined pattern of protrusions of a covering layer. As an example, a patterned coating layer can be applied by first forming an essentially homogeneous coating layer, which is then partially removed by means of lithographic and etching techniques, leaving nanometer sized protrusions in that layer. As a next step, the surface provided with these structures is modified by selectively removing protrusions by means of a micro-device. Such a micro-device can be formed in a similar way to a scanning probe microscope (SPM) tip. The presence or absence of a protrusion may represent a readable data bit information.
According to the method for manipulating a surface structure, one starts from a surface with a first structure of protrusions. The protrusions may be placed to form a pattern that is produced, for example, by lithographic techniques. As an alternative, the structure of protrusions may comprise a different, non-periodic structure of protrusions or even consist of a single protrusion. In a next step, protrusions are removed by applying a pre-defined force.
For the method for structuring a surface or for manipulating a surface structure, the pre-defined force may, according to a special embodiment, be actively influenced by a further parameter next to the protrusion size and the interface materials: The surface may be covered by a fluid other than air, e.g. a liquid, even immersed in a fluid. It has been found that the force for removing a protrusion strongly depends on chemical properties of such a fluid. As a trend, fluids with a high electron and proton donating capacity and fluids with small molecules tend to reduce the force for removing protrusions whereas with large, chemically inert molecules the required force is enhanced. This special embodiment, if applied on a storage device, provides an additional degree of freedom for designing a storage device according to particular needs.
A micro-device for removing protrusions can also be used to probe the structured surface and to read data. More generally, data are usually read by probing the topography of the structure produced according to the invention. If the material of the substrate and the protrusions differ from each other in physical or chemical properties, such as their magnetic or electric properties, the surface structure can also be probed using differences in these physical or chemical properties between the protrusions and the other parts of the surface.
In a preferred embodiment of the present invention, silicon is used as a substrate and silicon oxide as the protrusion material. It has surprisingly been found that the Si/SiO2xe2x80x94interface is very well suited for the purpose of forming a predetermined breaking point. The force needed to remove the oxide protrusions is relatively small compared to the force needed to break a pure silicon oxide or silicon structure and very well defined, so that the structuring process can be optimally controlled.
In another preferred embodiment of the present invention, the predetermined breaking point is not an interface but is given by the geometrical shape of the protrusion. For example, the predetermined breaking point may be the base of a column-like protrusion. The protrusion, in this case, may or may not be made of the same material as the substrate.
Also a combination may be envisaged: A protrusion may comprise a first predetermined breaking point given by an interface and a second predetermined breaking point, which is due to its geometrical shape. For example, a column-like protrusion may comprise two proportions, one being of Si and the other one of SiO2, the Si/SiO2 interface being a first predetermined breaking point, and the base of the protrusion being the second predetermined breaking point.
The force needed to remove a protrusion at an interface is different from the force needed to remove the protrusion at the geometrically formed predetermined breaking point. Therefore, by controlling the breaking force at least two information stages can be implemented on a single protrusion.
According to further preferred embodiment, substrate and protrusions are both of the same material, namely of silicon. An ultrathin oxide interface layer then forms the interface. Such an interface layer may, during an etching preparation step, further serve as etch stop. Ultrathin layers of silicon oxide embedded in silicon are nowadays available on the market due to the progress made in the SOI (silicon on insulator) technology.
Of course, any known or yet to be discovered substrate/coating layer material combination can be used, as long as a well-defined interfaces or geometrical shapes are formed. Different materials in the sense of this description also include one material with different additives or dopants or one material in different modifications such as carbon as diamond, graphite, amorphous carbon, etc.
An advantage of a surface or a data storage medium fabricated according to the present invention is that the structure or medium is extremely stable. There are, for example, no volatile materials or electronic or magnetic features that can be easily manipulated. If, for the reading process, a probe similar to an AFM tip or an array of AFM tips is used, the protrusions are essentially not altered at all. The information can thus be read arbitrarily often. If the appropriate materials are chosen, the storage medium according to the invention is thus a well suited medium for WORM (write once read many times) applications. Further, once the information is written on the storage medium, it cannot be altered any more except by removing other protrusions as well, i.e. by destroying the medium. The structure or information is, so to say, xe2x80x9cchiseled in stonexe2x80x9d. The method and storage medium according to the invention is thus extremely well suited for applications where security is a major issue.
For instance, the method and the storage device may be based on an array of micro-devices as disclosed in U.S. Pat. No. 5,835,477. Compared to existing storage devices, a significant improvement with respect to data access rate results.
According to a special embodiment, a pattern of protrusions is provided where each protrusion comprises a plurality of predetermined breaking points. The predetermined breaking points are configured so that, if a force is applied onto the protrusion, breaking off is done at one predetermined breaking point after the other. In this way, a storage medium can be altered to form a WSRM (Write Several times Read Many times) medium.
The structuring method according to the invention further gives rise to progress in investigation methods as well. It allows the investigation, for example by means of SPM probes, of physical or chemical properties of protrusions, such as the mechanical stability, friction erosion, etc. In addition, and possibly even more importantly, this method opens up new possibilities in the field of nanotribology and the studying of interfaces in a submicron range, which is of growing interest, i.e. for micromechanical systems. For example, by systematically measuring forces to be applied to break off a protrusion, a new access to interface adhesive forces and stability as well as interface sharpness is gained.